The invention relates to a new and improved gas or vapor alarm system containing at least one heatable and gas-sensitive sensor element and an evaluation or processing circuit.
In present times the environment is endangered and contaminated to a progressively increasing extent, and thus, organic living things or life must be increasingly protected from toxic and explosive gases or chemical vapors. Such protection is of particular importance and urgency in the chemical industry, traffic installations, such as garages or tunnels and in heating installations. Such protection, however, is not feasible without a preceding warning. Metal oxide semiconductors have become fairly well accepted as sensors suitable for such warning purposes. The metal oxide semiconductors (MOS's) react in their heated state, by virtue of a conductivity change thereof, to toxic and/or reducing gases or vapors as well as to water present in their environment. The change in conductivity is utilized as an indication of the integral gas or vapor concentration which is present.
Also, metal oxide beads impregnated with a catalyst have been used for the detection of explosive vapors or gases. The temperature increase, generated by combustion in the environmental air, at the heated surface of the bead is measured and thus serves to establish the presence of combustible gases.
It can be further recognized from research reports that gas sensor elements, like the so-called GAS FETs or CHEMFETs, are being developed on the basis of silicon technology and are supposed to be particularly suited for detecting explosive gases, such as hydrogen, or toxic gases like carbon monoxide, see NTG Fachberichte, Volume 79, 1982.
It has also been mentioned that pyroelectric elements fabricated, for example, of LiTaO.sub.3 are sufficiently sensitive to be able to detect temperature variations generated by the desorption of gases from the surface of such sensor element (Chemically Sensitive Devices, Elsevier, 1980).
The sensor elements given here as examples are operated at a constant elevated temperature which is determined by the response time and the cooling time, on the one hand, and by the gas to be measured, on the other hand. While already by virtue of the selection of the value of the constant temperature some preference may be obtained for the detection of individual gases, the sensor elements operated in such manner do not give any information as to the nature of the gas component or the chemical vapor. In such case the sensor element is considered to have a wide or broad band response behavior. For example, certain harmless concentrations of hydrogen produce a change in the electrical conductivity of a metal oxide semiconductor gas sensor element which is of the same magnitude as that which would be produced by a high and dangerous concentration of methane gas. A result thereof is undesirably signalling or indicating a false state-of-danger which frequently entails rather high consequential costs due to, for instance, unnecessary evacuation of an area, production interruption and so forth. There is therefore the desire to selectively detect individual dangerous gases. In practice this requirement has been fulfilled by manufacturing specific gas sensor elements for a certain gas like, for example, a selective sensor element for hydrogen sulphide or for a certain vapor or by equipping a metal-oxide semiconductor with preceding or upstream arranged filters for blocking the access of undesired gases. It is a disadvantage of these techniques that by virtue of such absolute selectivity other components are not detected which are simultaneously present in the environment. Heretofore there thus prevailed the limitation that there was carried out detection of only a single individual gas or vapor component in the environment. Other possibly occurring components were totally neglected. Thus, poisoning phenomena or explosions unfortunately have been known to occur without any response from the alarm system.
In a method for determining the content of a carbon monoxide component in a gas mixture as known, for example, from German Patent Publication No. 2,313,413, published Mar. 2, 1978, a metal-oxide semiconductor is heated to a predetermined temperature by means of a heating wire. After an uncontrolled cooling to room or ambient temperature there is accomplished the carbon monoxide measurement which is differentiated and integrated in a subsequently connected electronic evaluation or processing circuit. The evaluation or processing only occurs in time intervals during which no measurement is accomplished. In this method it is a disadvantage that during the cooling period various gases can settle at the surface of the semiconductor sensor element which then generate a signal similar to that generated by carbon monoxide. Also, at the most only one gas can be selectively detected using this method according to which the gas is detected at constant temperature which is about room temperature. Consequently, as stated hereinbefore, the required degree of selectivity is not ensured.